1. Field of the Invention
The invention relates to a method for producing tooling and to tooling apparatus for use in casting. More particularly, the invention relates to a method for producing a mold and to a mold for casting.
2. Description of the Prior Art
Casting methods require tooling such as a mold for containing the material to be cast and imparting a shape to that material. Common casting techniques include pressure infiltration casting, die casting and permanent mold casting.
Pressure infiltration casting is a process wherein a pressure differential is used to drive infiltration of a molten infiltrant into an empty mold to produce an unreinforced casting or into a mold containing a preform to produce a reinforced casting. Such a pressure differential can be established, for example, by evacuating the mold and placing the evacuated mold in a pressure vessel where a pressurized gas provides the pressure differential needed to drive the molten infiltrant into the mold. A gas-tight environment is required so that entrapment of gas pores or pockets does not create defects in the finished casting which can be provided by a mold container. Using pressure infiltration casting, it is possible to controllably cast highly loaded, continuously reinforced and discontinuously, i.e., particulate, reinforced composites to net shape and with a dimensional tolerance of .+-.0.0002 in with a surface finish of 4 microinches or 0.1 micron i.e., a superfinished surface like a mirror finish. Pressure infiltration casting can also be used to produce highly detailed unreinforced castings characterized by similar dimensional accuracy and smoothness. To achieve such results, machined graphite molds are typically utilized. Alloys cast with graphite molds include aluminum, magnesium and copper alloy castings. While graphite is easily machined and takes on a highly smooth finish, the graphite material and machining are expensive. Also, a graphite mold has a limited lifetime.
In pressure infiltration casting of reinforced composites, gas entrapment within the preform must be avoided so that gas pores or pockets will not create defects in the finished casting. A vacuum is isolated in the preform during pressure infiltration to eliminate such defects caused by entrapped gas. Since any gas leak will reduce the pressure gradient for infiltration, thereby resulting in incomplete infiltration and entrapped gas porosity in the finished casting, conventional, porous investment casting compounds cannot be used in pressure infiltration processes.
Permanent mold casting is a process which is described in ASM Metals Handbook, Ninth Edition, V. 15, page 275, as "gravity die casting, a metal mold consisting of two or more parts . . . repeatedly used for the production of many castings of the same form". Die casting is a process, as also given by ASM Metals Handbook, Ninth Edition, V.15, page 286, "characterized by a source of hydraulic energy that imparts high velocity to molten metal to provide rapid filling of a metal die. The die absorbs the stresses of injection, dissipates the heat contained in the metal, and facilitates the removal of the shaped part in preparation for the next cycle. The hydraulic energy is provided by a system that permits control of actuator position, velocity, and acceleration to optimize flow and force functions on the metal as it fills the cavity and solidifies." Steel molds are conventionally used in die casting and permanent mold casting processes. Materials die cast and permanent mold cast in steel molds include alloys of aluminum, magnesium and zinc. Machined graphite molds have a lower coefficient of thermal expansion than steel molds and produce castings characterized by higher tolerances at a lower cost than steel molds of comparable tolerance. However, graphite molds oxidize above 400 C. and must be coated, resulting in a shorter lifetime for the graphite molds than for comparable steel molds which typically have lifetimes of approximately 40,000 castings for die casting and up to 100,000 castings for permanent mold casting. When large numbers of parts are being cast, steel molds are, therefore, more economical than graphite molds for die casting and permanent mold casting.
When large numbers of parts are being cast by any of the already-described processes, mold properties and production become key factors determining the economics of the casting process. Thus, there exists a need for a semi-permanent mold made of a mold material conformable to the fine details of a part shape and also characterized by sufficient green strength to allow removal of the mold material from the pattern for the part shape. The mold must undergo curing and outgassing without distortion or excessive shrinkage. The cured, finished mold must be strong and shock resistant so that it can be used for many castings. Also, high mold thermal conductivity is desired to enhance the solidification rate in pressure infiltration, die casting and permanent mold casting processes. A need also exists for a gas-tight standard investment compound mold for use with a pressure infiltration process for production of reinforced composites.